As such a clamping apparatus, for example, one described below in Patent Document 1 is conventionally available, with the following structure.
A plurality of reference pieces are attached on an upper surface of a jig fixed on a bed of a machine tool. A clamp reference surface for receiving a worked surface of a work piece is formed on an upper surface of each of these reference pieces. A jet hole communicatively connected to a compressed air source is opened upwardly on the clamp reference surface.
Then, when the work piece is clamped, compressed air jetting out upward from the jet hole is used to clean the worked surface of the work piece, thereby preventing swarf from being caught between the worked surface of the work piece and the clamp reference surface.    Patent Document 1: Japanese Published Unexamined Patent Application No. S54-17580